Method of making plywood



Patented Feb. 25, 1941 PATENT OFFICE METHOD OF MAKING PLYWOOD James V.Nevin, Abei'deen, Wash.

No Drawing. Application April 2, 1938, Serial No. 199,581

2 Claims.

This invention relates to a method of making hot pressed plywood,wherein the constituent wooden plies are permanently bonded togetherwith a resin bonding agent. More specifically, the invention relates tothe coating and humidification control of the stacks of veneer sheetsthat are mounted in the hot press for forming plywood panels.

One of the major problems in the production of hot pressed plywood isthe regulation of the moisture content of the wood and of the adhesivematerial during the hot pressing operations. If an excess of moisture ispresent, steam explosions and surface checks are very likely to occur.If, on the other hand, the moisture content is below a desired minimum,the binder or adhesive .cannot function properly to bond the plies orveneers together.

In my Patent No. 2,068,759, there is described and claimed a method ofmaking plywood wherein successive coats of an aqueous solution of aresinous partial condensation product are applied to a core sheet. Thecore is dried after each coating to a moisture content of not over 4%.The final dried coating is then humidified to impart to the coatingitself a moisture content ,of from 15% to 40%. The thus treated andcoated core sheet is sandwiched between the veneer sheets to make up astack or pack of sheets for a plywood panel. The resulting stack issubjected to a heat and pressure treatment in a hot press forpermanently uniting the sheets.

I have now found that the humidification treatment of the dried coatingsprior to insertion in the hot press, can be eliminated. I have alsofound that it is desirable to utilize at least three coatings of theresinous binder solution. The first two coatings are dried while thethird coating is applied to the core sheet prior to its insertion in thehot press, to impart the desired moisture content of from 15 to 40 percent along the glue line.

The binder used for making the products of this invention is an aqueoussolution of a partially condensed meta-cresol-formaldehyde resinca/pable of being set into a hard infusible mass upon the application ofheat and pressure.

It is therefore an object of this invention to provide an improvedmethod of making hot pressed plywood that avoids the necessity forhumidifying the plywood veneer sheets prior to their insertion in thehot press.

Another object of this invention is to provide a process of making hotpressed plywood wherein a solution of a thermal setting bonding agent isutilized to impart a desired moisture content at the glue lines betweensheets of veneers during the hot pressing thereof to set the bondingagent.

A further object of this invention is to prevent steam explosions andchecking in plywood panels 5 during the hot pressing operation thereof.

Other and further objects of this invention will become apparent tothose skilled in the art from the following detailed description.

Veneers, used in the manufacture of hot pressed plywood according tothis invention, are prepared in the usual way by peeling wooden logs ona lathe. Redwood or Douglas fir "peeler logs are desirable for formingveneer sheets. It should be understood, however, that the sheets can beformed by a slicing operation instead of a peeling step.

The veneer sheets are dried to a moisture content of about 2 to ti /2%.The moisture content of the sheets should not exceed 4%. The driedveneer sheets should be stored in dry atmospheres.

According to this invention, the core sheets or cross banding veneers tobe used in forming the plywood panels are coated on both sides thereofwith an aqueous solution of a meta-cresol-formaldehyde partialcondensation product. This product is made by the exothermic reaction ofmeta-cresylic acid (meta-cresol) and formaldehyde in the presence of analkali metal hydroxide condensing agent. The reaction is allowed toproceed exothermically until somewhat constant temperatures of around208 degrees F. are reached. Further reaction is then arrested by dumpinginto the reaction mass an aqueous solution of sodium hydroxide. Theresulting material is a true aqueous solution of ameta-cresolformaldehyde resin that is soluble in water in allproportions.

It is preferred to carry out the reaction between the meta-cresol andformaldehyde in the presence of an alkali metal hydroxide equivalent tonot less than 10% of sodium hydroxide based on the weight of themeta-cresol. It is also preferred to arrest the reaction with a weakalkaline aqueous solution containing an amount of alkali metal hydroxideequivalent to not less than 6% of sodium hydroxide based on the weightof the meta-cresol. The aqueous solution of the metacresol-formaldehydecondensation product is soluble in water in all proportions and isadjusted to form a standard,30% solution.

The dried c'ore veneer sheets are then given a first coating of the 30%resin solution on both surfaces thereof by passage of the sheets througha coating machine having top and bottom coating rolls. A coating of 20pounds of resin solution per thousand square feet of core surface isapplied. The thus coated cores are dried in a drying tunnel to reducethe moisture content of the coating and of the core ply to not more than4%.

The dry coated cores are next passed through a second coating machinefor applying a 30 pound coating of the same resin solution thereon. This30 pound coating means that 30 pounds of resin solution are spread oneach thousand square feet of core surface coated. The second coating onthe cores is then dried to reduce the moisture content of the coatingand of the cores to not more than 4% of the weight of the coated Thefirst lighter coating penetrates appreciably into the surfaces of thecore sheets. When this coating is dried, however, the surfaces of thecore sheets are sealed, so that the second heavier coating remains onthe surfaces.

The double coated and dried core sheets can now be stored in a dryatmosphere until it is desired to lay the same up into plywood panels.This storing, of course, is not necessary but is convenient in that anumber of cores can be prepared ahead of time for subsequent panelformation in the hot presses. Thus, sufllcient cores can be prepared inone work shift to supply the hot presses through three shifts.

The double coated and dried core sheets next receive a light thirdcoating of the 30% resin solution thereon. This third coating isequivalent to pounds of 30% resin per thousand square feet of coresurface coated. The triple coated core sheets are next laid up intostacks between uncoated veneer sheets. The stacks are allowed to standfor thirty minutes to one hour before insertion into the hot presses, sothat the third coatings on the core sheets have sufficient time topenetrate into the dried coatings. The total amount of dry resindeposited on the core sheet is 30% of the 65 pounds of resin solutionapplied in the three coatings, or 19.5 pounds.

The first two coats containing altogether 15 pounds of resin are driedto a moisture content of 4%, leaving therefore .6 pound of residualmoisture in the coatings. The final coating adds to this moisture, 10.5pounds of water which remains in the coating, so that the finalcomposition of the triple resin layer is 11.1 pounds of water togetherwith 19.5 pounds of dry resin. In other words, the final moisturecontent along the glue line amounts to about 36% The hot presses arepreferably operated at temperatures around 330 degrees F. to set themeta-cresol-formaldehyde resin into a hard, infusible and insolublebinder permanently welding the sheets together. If the veneer sheets areformed of Douglas fir, pressures of about 175 pounds per square inch areused in the hot presses. If the veneer sheets are redwood, lowerpressures of around 130 pounds per square inch are used.

The total resin content of plywood panels prepared according to thisprocess is about 19 pounds of dry resin per thousand square feet ofresin coated surface.

After the hot pressed panels are removed from the presses, the same areimmediately inserted in humidifiers before they cool down. Thishumidifying treatment imparts a desired moisture content into the panelsof between Band 12% to prevent subsequent warping of the panels.

From the above description, it should be understood that this process ofmaking plywood includes the triple coating of core sheets or crossbanding with an aqueous'solution of a thermosetting resin. The first twocoatings are dried to a moisture content of not over 4%. The thirdcoating is then applied to the double-coated dried cores and is allowedto'penetrate into the first two coatings on the cores, This thirdcoating supplies the desired amount of moisture so that the binder willfunction to give a good "stick and will be driven into the surfaces ofadjacent uncoated veneer sheets in the plywood panel. At the sametime,this third coating does not supply sufilcient moisture at the glue linesfor causing steam explosions or checks during the hot pressingoperation.

I am aware that numerous details of the process may be varied through awide range without departing from the principles of this invention, andI, therefore, do not purpose limiting the patent granted hereonotherwise than is necessitated by the prior art.

I claim as my invention:

1. The method of making hot pressed plywood panels which comprisesdrying a core sheet of wood to a moisture content below four per cent,coating the broad faces of the dried core sheet with a thirty per centaqueous solution of a thermo-setting cresylic acid-formaldehyde resin,drying the coated sheet to leave a substantially dry film of thethermo-setting resin on each face thereof, applying a second coating ofsaid aqueous resin solution over the dried films, again drying the coresheet to dry the second coating into substantially dry films, applyingto the coated core just prior to laying the same between veneer sheets athird coating of said aqueous resin solution over the dried film thereonin an amount sufficient to impart a moisture content of from 15 to 40per cent along the glue line, sandwiching the triple coated core betweenthe wooden veneer sheets without drying the third coating to form astack, allowing the wet third coating to penetrate into the driedcoatings and veneer sheets, and subjecting the stack to heat andpressure for uniting the core and veneers.

2. In the process of making hot pressed plywood from veneer sheets and acore sheet which has both broad faces thereof covered with substantiallydry films of thermo-setting resin binder, the steps which comprisecovering, just prior to laying the core sheet between the veneer sheetsto form a stack, the dry films on the core with films of an aqueousthermo-setting resin binder in an amount sufficient to impart from 15 to40 per cent moisture along the glue line, laying the thus coated corebetween the veneer sheets without drying the aqueous films, allowing thestack to stand to permit penetration of water from the aqueous filmsinto the dry films and into the veneers, and uniting the core andveneers by subjecting the stack to heat and pressure for thermo-settingthe binder.

JAMES V. NEVIN.

